Influence of the process pressure and O₂ addition in the crystallinity and optical band-gap of ZnO thin films obtained by RF magnetron sputtering

Full text: Zinc oxide (ZnO) is a semiconductor material with an optical band gap in the range of 3.0 eV to 3.4 eV at 300 K. The optical properties can be controlled by changing the stoichiometry between the zinc and oxygen and by the addition of other chemical elements to the crystalline structure through doping. The objective of this work was to develop deposition recipes in order to have optimally controlled crystalline phase and optical properties (high transparency), by changing the process pressure and composition of process gas by using oxygen as additive on argon plasmas. In this work in particular we studied films of pure ZnO composition, controlling the stoichiometry between Zn (zinc) and O (oxygen) by changing the Ar:O₂ ratio used as plasma source gas on a high purity (99.99%) ZnO target (5 cm diameter). The depositions were preformed on a 13.56 MHz parallel plate RF magnetron sputtering tool with 4.5 cm target-to-substrate distance. All films analyzed have thicknesses monitored to be in the range of 200 nm, showing high optical transmittance (larger than 98%) in the UV-visible range (400 nm to 800 nm). The optical band gap of all obtained films was in the range of 3.2 ± 0.1 eV, in good agreement with the values reported in the literature. The X-ray diffraction (XRD) patterns along with SEM imaging showed a strong influence of the process pressure and the proportion of oxygen as a process gas additive on the morphology and on the crystallinity of the deposited films. The crystallinity of the film (monitored by the intensity and FWHM of specific 34 deg 2-theta diffraction peaks) improves with increasing pressure (in the 1 Pa to 3 Pa range), but decreases when increasing the relative amount of O₂ in the Ar+O₂ gas mixture used during the deposition (in the 0 to 25% range). (author)